The achievement of correct fuel supply to the cylinders of an internal combustion engine is a difficult problem the acuteness of which has become particularly great because of the pollution which accompanies any defective carburation. Incomplete solutions, which make it possible to obtain the desired power at the expense of increased consumption, are no longer acceptable at the present time because of the increased content of harmful gases which they produce in the exhaust emission.
The object of the present invention is an improved carburetor in which the petrol supply conditions are so established that the homogeneity and the degree of atomization of the mixture reaching the cylinders are greatly improved, these two conditions being fundamental for good carburation.
In known carburetors, for example those of the fixed jet type such as that shown in FIG. 1 of the accompanying drawings, there are always provided a petrol jet 1 and an air jet 2 which effect the formation of an emulsion, which can be considered as a compressible medium in which the waves are propagated at a speed close to that of sound in air, that is to say 300 meters per second. (In the case considered the wave in question is a negative pressure wave.)
The negative pressure wave produced by the suction of the cylinders is therefore propagated in the induction manifold 3 at about 300 meters per second and, as a rough approximation, the same speed can be accepted for the propagation of this wave, at the level of the diffuser 4, through the spray nozzle 6 in order to reach the petrol level 5 and the initiate the stream of petrol. By the expression "petrol level" must be understood the level assumed by the petrol under operating conditions. The distance usually adopted in carburetors of this kind between the center of the stream of air in the diffuser and the point 5 where the mean petrol level is established is of the order of 35 to 40 mm, and in all cases is greater than 30 mm.
In normal petrol internal combustion engines the priming of the main circuit is established at about 1500 r.p.m. At this speed the duration of one rotation of the crankshaft through 1.degree. is 1/9000 second and the time taken by the negative pressure wave during the induction phase to reach the petrol level from the diffuser is at least equal to ##EQU1##
At 1500 r.p.m. this corresponds to a crankshaft rotation of at least about 1.degree. .
It is then necessary that the petrol should reach the neck of the diffuser 4; since the difference in pressure between the neck of the diffuser and the petrol level is low (of the order of 100 g at most) the speed of circulation of the emulsion will be substantially lower than the speed of circulation of the driving wave.
The time necessary for the petrol to travel the distance separating the center of the diffuser 4 from the level 5 will therefore correspond to an additional magnitude of crankshaft rotation additive to the previous mentioned crankshaft rotation.
Starting from the moment when the system is primed, the negative pressure wave is propagated through an emulsion the speed of which slows down the propagation of the negative pressure wave, so that this minimum figure is further increased.
Furthermore, the petrol level becomes lower, the higher the speed of the engine, so that the phenomenon is intensified.
It can therefore be taken as certain that the emulsion flow wave will lag behind the negative pressure wave by a time greater than the duration of crankshaft rotation during transmission of the negative pressure wave. The negative pressure wave corresponds to the flow wave of the air introduced into the engine. Consequently, the air introduced during initial crankshaft rotation will not be carburetted and the flow of petrol will continue while the flow of air will be interrupted by the falling of the suction valve.
This delay is intensified by the fact that at the moment when the air flow is interrupted in the diffuser atomization becomes very poor. Thus, in the last part of the air and petrol flow wave passing through the zone between the diffuser and the inlet valve, not only will the mixture be rich, but it will be poorly or only slightly atomized, so that bad carburation will result.